In order to optimize the investment in optical fiber links, it is desirable to increase the capacity of said links. This can be achieved by increasing the Spectral Efficiency (SE) of the signals transmitted on said links, namely by using more efficient modulation formats for the transmitted information. This can be used in conjunction with Wavelength Division Multiplexing (WDM). Optical communication systems with transmission rates up to 10 Gb/s mainly utilize On Off Keying (OOK) for modulation, where the information is coded on two amplitude levels of the lightwave signal. Besides, higher capacity systems utilize the modulation scheme based on Quadrature Phase Shift Keying (QPSK), which codes the information on four phase levels. Therefore, two binary bits can be coded per transmitted symbol. This is illustrated in the non patent literature 1 (NPL1). In this manner, the necessary bandwidth of the optical spectrum required to transmit information is used more efficiently, enabling the transmission of more information on a fixed bandwidth.
Another way to increase even more the spectral efficiency per transmitted channel, and therefore the link capacity, is to use Quadrature Amplitude Modulation (QAM), where symbols are coded on phase and amplitude levels, and are organized as a combination of multi-level amplitudes in quadrature phase. An example of QAM system is disclosed in the non patent literature 2 (NPL2). In NPL2, the modulation format is 16QAM, where the information is coded into 16 levels, that is, 4 binary bits per symbol. This enables to increase the spectral efficiency as compared to QPSK.
Information in QAM format can be modulated with an optical IQ modulator (In phase—Quadrature phase modulator), sometimes called Cartesian modulator, vector modulator, Dual Parallel modulator or nested modulator depending on the sources. In an IQ modulator, the electric signals drive two independent Mach-Zehnder devices, which can be called children Mach-Zehnder Modulators (MZM), or nested MZM depending on the sources. The children MZM modulate the phase and amplitude of the same optical carrier wave. The phase in one of their outputs is relatively delayed by 90 degrees before being recombined. The phase delay between the outputs of the children MZM can be called an angle of quadrature and is ideally 90 degrees, modulo 180 degrees. Such IQ modulators are used in NPL2 for QAM format and also used in NPL1 for QPSK modulation. Such IQ modulators offer an efficient and proven way to perform QAM format.
More specifically for QAM format, the electrical signals used to drive the IQ modulators are multilevel signals, as opposed to binary signals used for QPSK modulation, such as in NPL1. For instance in NPL2, said multilevel signals are 4 level signals, whose symbols are generated by two binary bits of independent data lanes. In the case of NPL2, two bits, the Most Significant Bit (MSB) and the Least Significant Bit (LSB) are combined by a scheme, which is called by some sources as Power Digital to Analog Converter (DAC). The scheme of power DAC is detailed in non patent literature 3 (NPL3) for two binary bits combined in a 4 level signal. The LSB signal is attenuated, when compared to the MSB with an attenuator and both signals are added with a power divider used as a power combiner. By extension, higher indices QAM format can be obtained with power DAC generating multilevel electrical driving signals with higher level count. For instance, in non patent literature 4 (NPL4), 64QAM is generated with a Power DAC, and 64QAM signal allows higher spectral efficiency. The Power DAC used in NPL4 is described in non patent literature (NPL5) and has the same functions of the Power DAC used in NPL3, with the addition of binary signal regeneration, DC offset and active amplification. In the Power DAC of NPL5, the amplitudes of the three binary signals are adjusted so that after combining said binary signal, a 8-level signal is produced with proper level voltages. Power DAC can be made of discrete RF (Radio Frequency) components, such as NPL3, or can be integrated onto a single chip, as NPL5. The advantage of Power DAC over other schemes used to generate QAM is that they are made with available and proven components and technology, enabling lower costs and better reliability as well as sooner availability, and require less power consumption.
Besides, it is known that there is a drift of DC (Direct Current) bias in IQ modulators due to variation of the temperature or ageing of the device. There are three types of affected biases, that is, the DC biases of each of the two children MZM and DC bias used to set the angle at quadrature. This is already known about QPSK modulation and also known about QAM format if it uses a modulator having the same structure. Drifts in biases result in incorrectly setting the modulator, which causes a degradation of the transmitted signal, and therefore results in degradation of the received signal quality or in worst cases the impossibility to decode the received signal. This problem is likely to be revealed in the characterization tests of the modulator at the production stage or at the assembly stage of the transmitter in which it is used, and when the modulator is used. This problem is solved for OOK, Phase Shift Keying (PSK) modulation and QPSK by using Auto Bias Control (ABC) circuits, which controls the biases of the modulators and to compensate for the DC bias change. In this manner, ABC technology can manage the drift of DC bias drift of IQ modulators, enabling correct modulation and optimal
For instance, patent literature 1 (PTL1) discloses a transmitter, which comprises an ABC scheme, which can control biases of an IQ modulator, which emits optical signal modulated with QPSK or even QAM format. The control mechanism of the ABC circuit relies on a low frequency dither or dither signal imprinted onto the DC bias to be controlled or onto the electrical signal driving the IQ modulator. Such methods relying on low frequency dither signals for ABC control are cost and size effective and have proven efficiency for modulation formats up to QPSK.